The invention concerns generally processes for the fabrication of a micromechanical structural element from a substrate, notably using semiconductive material by micromachining techniques.
More particularly, the invention concerns a process for fabricating a generally elongated structural element having at least one wing, which is thin with respect to its other dimensions, and extends in a direction substantially perpendicular to the substrate.
Such structural elements are found notably in applications concerning mechanical microstructures and particularly in microstructures such as X/Y tables, activators or the like.
As in classical mechanics, micromechanics use structural elements having different forms and sections adapted to the mechanical properties desired in an intended application.
A particular problem always arises in micromechanics relating to the fabrication of generally elongated elements such as beams or likes structures.
FIGS. 1 to 4 illustrate the difference stages of a conventional process used to fabricate a generally elongated structural element, in this instance a beam having a rectangular section.
According to this process, a substrate 1, for example of a semiconducting material, has an oxyde layer 2, called a sacrificial layer, (FIG. 1) applied to its surface. A layer 4 of material is then deposited, this layer being intended to form the beam 6 and having a thickness E defining the height of the beam (FIG. 2). The shape and the dimensions of the plane of the beam 6 are thus defined in the layer 4 by a classic lithography process and the layer 4 is etched according to the shape and dimensions thus defined (FIG. 3). The part of the layer 2 in the region of and under the beam 6 is finally submitted to a selective and isotropic etching in order to liberate the beam 6 (FIG. 4) which rests anchored, at least at one of its extremities, to the substrate 1.
Many microstructures exist whose operation requires the formation of structural elements having at least one wing which is thin relatively thin compared to its other dimensions and notably when compared to its dimension (its height) which extends in a direction perpendicular to the substrate, and thus has a great flexibility in a direction parallel to the plan of the substrate whilst also having a great rigidity in a direction perpendicular to the plan.
In fact, when one wishes, for example, to fabricate a suspension structure for an X/Y table in polysilicon which can be activated electrostatically, such as that which is described in the publication entitled "Micro Electro Mechanical Systems '92" Travenmunde (Germany), on 4-7 Feb. 1992 by V. P. Jaecklin, C. Linder, N. F. de Rooij, J. M. Moret, R. Bischof and F. Rudolf, it is desirable, in order to improve its operating characteristics and notably the forces therewithin, to realize long, very straight beams in thick layers. In other words, if the table extends in an X/Y plane and the beams forming its suspension structure have their width, length and height respectively along the axes X, Y and Z (the axis Z being perpendicular to the X/Y plane), the relation of the height to the width, known generally by the term "aspect ratio", must be as large as possible and preferably in the order of 10/1 for a width in the order of 0.2 .mu.m.
However, the aspect ratio that can be obtained by classical processes such as those described above is in the order of 2/1 for a minimum width of about 1 .mu.m.
This aspect ratio is limited by the practical impossibility of realizing a completely anisotropic etching of the beam material (etching uniquely along the Z axis and no etching along the X and Y axes). The etching of great heights leads consequently to beams whose section is trapezoidal and whose mechanical properties vary along their height. Thus, this acts against the optimal operation of the table by increasing the restraints of flexion and therefore the tensions necessary to operate the table.
Furthermore, it can be easily understood that even if, in principle, the classical processes allow the formation of structural elements having non rectangular sections, such as sections in the form of a U, L or a combination of U and L at the price of an extremely complex fabrication, these processes do not allow, taking into account the above, the realization of structural elements having several wings of rectangular section connected together with each of the wings having a small thickness, for example 0.2 .mu.m, whatever may be their orientation with respect to the plane of the substrate.